nodes: made changes to the node system to allow for context sensitive shader code

Primarily implemented for the math node so a new shader can be generated and
compiled based on what inputs the node is receiving.

app/node/audio/volume/volume.cpp

@@ -36,6 +36,11 @@ QString VolumeNode::Description() const
   return tr("Adjusts the volume of an audio source.");
 }
 
+Node::Capabilities VolumeNode::GetCapabilities(const NodeValueDatabase &) const
+{
+  return kSampleProcessor;
+}
+
 NodeInput *VolumeNode::ProcessesSamplesFrom() const
 {
   return samples_input_;